WASHINGTON — As the Senate cast its first votes on immigration measures Thursday, President Donald Trump again threatened a veto, this time against a new bipartisan proposal that would legalize Dreamers but guarantees $25 billion for a border wall.

The Senate was poised to vote on four measures, including the Trump-backed bill proposed by Sen. Charles Grassley (R-Iowa) and the new bipartisan measure led by Sen. Susan Collins (R-Maine.) — turning a week meant for a free-for-all debate into a four bill showdown.

Ahead of the votes, the Trump administration launched an all-out attack on the much-anticipated Collins compromise crafted and approved by eight members from each party caucus, raising doubts any bill set for a vote would get the 60 votes needed to pass.

Collins said Senate Republican leaders had stacked the deck against her group’s measure by putting the Grassley bill last in the series, giving it the best chance to pass.

At a midday news conference, Collins and her colleagues defended their bill, and acknowledged it is narrow measure. They said a bigger immigration debate should come after fixing the status of young immigrants, often called Dreamers, who were brought illegally to the United States as children.

“It is not a comprehensive immigration bill,” Collins said at a news conference with all the members of the group present after they announced they had settled on a proposal late Wednesday night.

“It is instead a good-faith effort, a bipartisan effort, to address some pressing urgent issues and to lift the cloud of uncertainty over the heads of young people who are contributing to our economy and our society and remove the risk of deportation after March 5,” she said.

The White House issued Trump’s veto threat, saying the administration “strongly opposes” the bill, shortly before the bipartisan news conference began. And the Department of Homeland Security harshly criticized the proposal on its website.

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The flashpoint was language at the end of the bipartisan measure that sets the priority for immigration enforcement on felons and other criminals for all noncitizens here without legal authorization who came through July 30, 2018.

The DHS said that clause “eviscerates the authority of the DHS to arrest, detain, and remove the vast majority of aliens illegally in the country.” A White House official in a press call called that provision “reckless” and “dangerous.”

But Collins said it was simply setting priorities, not banning immigration authorities from upholding the law.

She said that instead of “going after a chemistry professor who has been in this country for many years” the first priority should be “people who have committed felonies” and other crimes who represent a danger to the community.

Collins said the July 30, 2018 date, which a White House official said declared open season for noncitizens to slip illegally into the United States, is meant to be a deterrent, sending a message that after that date “we are going to focus resources on your deportation.”

The Trump-backed bill sponsored by Grassley offers 1.8 million Dreamers a 10- to 12-year process for gaining citizenship, provide $25 billion to build a border wall and restrict legal immigration.

The plan also gives give 1.8 million Dreamers a path to citizenship and provide the $25 billion for a wall over 10 years. But it bars the Dreamers from sponsoring their parents for U.S. residence, because they broke the law by being the United States illegally.

Senate Majority Leader Mitch McConnell (R-Ky.) once again pressured his own members and Democrats to support the Grassley bill, calling it the only legislation that can pass the Senate and House and get the president’s signature.

Schumer said, “President Trump, since he created the problem by terminating DACA last August, has stood in the way of every single proposal that has had a chance of becoming law.”

Schumer urged colleagues to support the bipartisan package, acknowledging both sides would find things they like and oppose in the bill.

“But we have to do our jobs today,” he said. “We have to rise above our differences, admit that no one will get everything they want and accept painful compromises that come with democratic government.”